In turbo-molecular pumps, there has been extensively used a construction in which a rotor having exhaust blades is supported in a non-contact manner by magnetic bearings. Its example is shown in FIG. 4 by way of a cross-sectional view. In this example, fixed blades 11 are arranged on an inner periphery of a case 10, and also a rotary shaft 13 having rotary blades 12 fixed to an outer periphery thereof is rotatably disposed within this case 10. Rotation is imparted to this rotary shaft 13 by an electric motor 14, and in its rotating condition the rotary shaft 13 is supported in a magnetically floated condition in a non-contact manner by two radial magnetic bearings 15, 16 disposed in proximity to an outer peripheral surface of the rotary shaft 13 and a pair of axial magnetic bearings 17 disposed respectively at upper and lower sides of a flange portion 13a formed integrally with the rotary shaft 13. Incidentally, in the drawing, 9 denotes an intake port, and 8 denotes an exhaust port.
Also, full complement ball bearing 21 with one deep groove and a pair of angular contact ball bearings 22 which are combined together in such a manner that their angular contact directions are reversed to each other are disposed as touchdown bearings within the case 10. These touchdown bearings 21, 22 are rolling bearings for protecting the rotary shaft 13 from damage caused by contact with the radial magnetic bearings 15, 16 and the axial magnetic bearing 17 at the time of stop of the rotary shaft 13 and at the time of occurrence of a control abnormality, and a clearance slightly smaller than a clearance between the rotary shaft 13 and each of the magnetic bearings 15, 16 and 17 is provided between the outer peripheral surface of the rotary shaft 13 and an inner peripheral surface of an inner ring of each of the touchdown bearings 21, 22. With this arrangement, in a condition in which the rotary shaft 13 is rotatably supported in a magnetically floated condition by the magnetic bearings 15, 16 and 17, the rotary shaft 13 is held in a non-contact condition relative to each touchdown bearing 21, 22. However, at the time of stop of the rotary shaft 13 or at the time of occurrence of a control abnormality due to an influence of an external force or the like, the rotary shaft 13 contacts the inner rings of the touchdown bearings 21, 22 and is rotatably supported by them before the rotary shaft 13 contacts the magnetic bearings 15, 16 and 17.
Incidentally, although the foregoing shows the example in which the inner rings serve as the rotary rings, there is known a structure in which a tubular rotor with rotary blades is used, and touchdown bearings are disposed within this tubular rotor. In this structure, outer rings of the touchdown bearings are disposed on an inner peripheral surface of the tubular rotor through a predetermined clearance, and the inner peripheral surface of the rotor contacts outer peripheral surfaces of the outer rings of the touchdown bearings at the time of stop of the rotor or at the time of occurrence of a control abnormality, so that the rotor is rotatably supported.
In the above touchdown bearings in the vacuum pump, in many cases, ceramics or stainless steel such as SUS440C is used as a material for rolling elements (balls), and similar stainless steel such as SUS440C or bearing steel such as SUJ2 is used. And, in view of the environment in which it is used, lubricating oil can not be used, and therefore a coating composed of any of various solid lubricants including as a representative example molybdenum disulfide is formed on at least one of the surfaces of these members contacting other members (see, for example, Patent Literature 1).
Here, in this kind of touchdown bearing, the durability of the coating of the solid lubricant whose representative example is molybdenum disulfide determines the durability of the bearing itself. Therefore, in order to enhance the adhesion of molybdenum disulfide to a substrate, a processing for forming microscopic pits and projections on a surface by shot blasting or the like is conventionally usually effected as an undercoat processing for the molybdenum disulfide coating.
With respect to balls, an Ag ion-plated coating or the like serving as a solid lubricant coating is adopted. Patent Literature 1: JP-A-2002-221226 Publication